1. Technical Field
The present invention relates to water-resistant optical fibers.
2. Background of the Invention
Typically, an optical fiber comprises a glass fiber, made from silica; and a coating at the circumference of the glass fiber, made from a coating resin. The coating prevents strength reduction. An ultraviolet-curable resin, particularly a urethane-acrylate type or an epoxy-acrylate type, is generally used as the coating resin for the optical fiber.
An optical fiber increases its transmission loss due to external stress and microbending caused by such external stress. To protect the optical fiber from such external stress, the optical fiber is usually coated with two layers (a soft layer and a hard layer) of material. For the inner layer, which directly contacts with the silica glass, a soft resin with low Young's modulus is used as a buffer layer (hereafter a primary layer); and for the outer layer, a hard resin with high Young's modulus is used as a protective layer (hereafter a secondary layer). Conventionally, a resin with 3 MPa or less in Young's modulus is used as the primary layer, and a resin with 500 MPa or more in Young's modulus is used as the secondary layer.
Such optical fibers are manufactured by the following process. First, a glass fiber is drawn from a silica glass preform by heating the preform in a drawing furnace. Then immediately afterward, a liquid ultraviolet-curable resin is coated onto the drawn silica fiber via a coating die and cured with ultraviolet light. The primary and secondary layers are created by this process. There are methods for coating the primary and secondary layers at the same time, and then simultaneously curing both; and there are methods for coating and curing the secondary layer after the primary layer has been coated and cured.
As the popularity of optical fibers is growing in recent years, the number of optical fiber cable applications is increasing. Because of the popularity, the environments where optical fiber cables are used have been diversified, and the long-term reliability required for optical fiber cables becomes stricter. With the situation as stated above, an optical fiber, which is less likely to increase its transmission loss when it is immersed in water for long period of time, is being considered. For example, Japanese Patent Application Laid-open No. 09-005587, discloses an optical fiber, which does not increase its transmission loss even when it is immersed in water for long periods of time due to strengthened adhesion between the primary layer and the glass fiber.
It is known that in an optical fiber, which increases its transmission loss when it is exposed to a high-humidity environment or immersed in water, delamination is observed in the boundary layer between the coating layer and the glass fiber. Delamination between the coating layer and the glass fiber occurs when the force applied to peel off the coating layer in a boundary face between the glass fiber and the coating layer is greater than the boundary face adhesive force between the glass fiber and the coating layer. When delamination occurs at the boundary face between the glass fiber and the coating layer, a force applied to the glass fiber becomes uneven. The unevenness in the force causes microbending and consequently the optical fiber increases its transmission loss.
The mechanism that causes the adhesive force between the glass fiber and the coating layer to be reduced when the optical fiber is immersed in water is inferred as follows. When the optical fiber is immersed in water or exposed to a high-humidity environment, moisture passes through the coating layer and reaches the boundary face between the glass fiber and the coating layer. Adhesive force exists at the boundary face between the glass fiber and the coating layer and, in general, comprises hydrogen bonds between glass fiber and a functional group in a resin, and chemical bonds from an adhesion accelerator (see for example, N. Akasaka et al., “Design of Optical Fiber Coating”, Proc. of 19th Australian Conference on Optical Fibre Technology (ACOFT), p. 375, 1994). However, it is believed that the hydrogen bonds are disconnected when water penetrates the boundary face between the glass fiber and the coating layer. As stated above, it is inferred that the adhesive force at the boundary face between the glass fiber and the coating layer is reduced by the disconnection of the hydrogen bonds.
Various optical fibers, which were less likely to increase their transmission loss when immersed in water, have been proposed. However, as shown in Japanese Patent Application Laid-open No. 09-005587, known methods to suppress transmission-loss increase by balancing the adhesive property of each boundary layer have limitations and these methods do not offer sufficient reliability.
With the situation as stated above, the purpose of the present invention is to provide an optical fiber, which suppresses its transmission-loss increase due to environmental or age deterioration, particularly when it is exposed to a high-humidity environment or is immersed in water.